1. Field of the Invention
This application relates to integrated circuit devices more particularly to integrated circuits utilized in generating clock signals.
2. Description of the Related Art
Clock sources typically utilize a resonator such as a crystal oscillator or surface acoustic wave (SAW) device. Precision in traditional clock sources utilizing crystal oscillators is determined by the accuracy of the cut of the crystal and the calibration performed after the cut. For example, frequency tuning may be achieved by sputtering gold after cutting the crystal. Fixed frequency sources such as crystals have typically provided better phase noise performance than the phase noise performance associated with variable frequency source such as, e.g., a voltage controlled oscillator (VCO). That is due, at least in part, to the fact that the variable elements (e.g. the varactor) associated with the VCO used to vary the frequency have higher losses than fixed elements such as the capacitors in a fixed source.
However, resonators typically have a limited optimum range due to manufacturing constraints. That is, it is hard to pull a crystal over a wide range. However, various applications have requirements for numerous frequencies outside the easy range for a resonator. Typically, a different frequency range will require a different resonator. Accuracy requirements vary for clock sources, but are typically in the parts per million (ppm) range.
In another aspect, clock signals are used by transmission systems to synchronize the flow of data. Such clock signals, particularly high speed clock signals, may include jitter, which should be managed to prevent bit errors. Jitter is the variation in clock output frequency from a desired output frequency and can occur for a number of reasons. Jitter may be caused by noise introduced into the system from any of a variety of sources including jitter present on a reference clock utilized in generating the high speed clock signals. It would be desirable to provide improved techniques for supplying high speed clock signals.
It would also be desirable to provide a clock source that meets accuracy requirements, allows the use of a resonator that is easy to manufacture and low cost, but can still provide a wide range of output frequencies and suitable phase noise performance.